Packaging of electronic circuits is the science and the art of establishing interconnections and a suitable operating environment for the circuits to process or store information. The packaging must reconcile and satisfy application requirements with respect to cost, performance, and reliability, as well as constraints imposed by the laws of nature and the properties of materials and processes. Generally, environmental conditions such as temperature extremes, high relative humidity, mechanical shock and vibrations, electromagnetic fields, electrostatic discharges, and nuclear radiation may contribute to failures of electronic packages.
Currently, there are many different shapes and sizes of semiconductor packages available, including laminated ceramic, pressed ceramic, and molded plastic packages. In accordance with the laminated ceramic technology, a semiconductor die is attached to a ceramic package having leads extending from a lead frame. Bonding pads on the die are connected to the leads using bonding wires. A cap is then soldered to the ceramic package, thereby sealing the die and inner portions of the leads within the package.
In pressed ceramic technology, a semiconductor die is attached to a lower portion of a ceramic package having leads extending from a lead frame. After the wire bonding procedure, a top portion of the ceramic package is soldered to the lower portion of the ceramic package to seal the die and the inner portions of the leads within the package. Ceramic packaging is relatively expensive and the ceramic material causes a relatively large inductance, thus slowing down the signal propagation through the device.
With molded plastic technology, a semiconductor die is configured for housing within a plastic package from which a set of leads will extend. FIGS. 1 through 5 and 6A-6C illustrate the general process flow for a plastic molded I.C. package assembly. FIG. 1 shows a conventional lead frame 10 having outwardly extending leads 12. Lead frame 10 is the central supporting structure of the package to which every other element is attached. Etched or stamped from a thin sheet-metal strip into a filigree of narrow beams that radiate from a center platform, lead frame 10 carries the chip throughout the assembly process and becomes an integral part of the package after molding.
An I.C. die 14 is attached to lead frame 10 with a conductive adhesive 16, as shown in FIG. 2. FIG. 3 shows I.C. die 14 electrically connected to the radiating beams (not shown) of lead frame 10 with fine-diameter bond wires 20 between bond pad 18 on I.C. die 14 and bond pad 19 on lead frame 10, bond pad 19 being electrically coupled to one of the leads 12. This assembly of I.C. die 14, bond wires 20, and lead frame 10 is placed in an injection molding device 22 that has a cavity 24 and an opening 26 for injecting a molding compound into cavity 24, as shown in FIG. 4. I.C. die 14, bond wires 20, and lead frame 10 are covered with the molding compound during the injection molding process, which is carried out at a high temperature. Conventional materials used for the molding compounds are novolac-based molding compounds.
Injection molding device 22 is pulled apart, leaving a plastic package 28 with leads 12 extending from plastic package 28, as shown in FIG. 5. Leads 12 are trimmed, formed, and tin-plated to complete the package. FIG. 6A shows leads 12 formed into butt joint leads 12a. FIG. 6B shows leads 12 formed into J-leads 12b. FIG. 6C shows leads 12 formed into gull wing leads 12c.
The various packaging structures and methods described above may be sufficient for a traditional I.C. die. However, in the field of biometric identification, e.g., fingerprint identification, where the surface of the I.C. die must directly interface with its external environment, e.g., the finger, such structures and methodologies are incompatible because the I.C. die is encapsulated in various ways in the package. Efforts have been made to build an I.C. die package that leaves open the surface of the chip. One example is the package described in U.S. Pat. No. 5,862,248 (hereinafter, the '248 patent) issued on Jan. 19, 1999, to Salatino et al. and assigned to Harris Corporation, which is incorporated by reference herein in its entirety.
The '248 patent describes an integrated circuit device having an opening exposing the integrated circuit die to the sensed medium, such as a finger, and related methods. In particular, the '248 patent describes a package that includes an integrated circuit die 54 mounted on a lead frame 50 during injection molding to form the body of encapsulating material of the package, as shown in FIG. 7. Electrical connections are made by bond wires 60 between bond pads 58 on I.D. die 54 and bond pads 59 on lead frame 50, each of bond pads 59 is electrically coupled to corresponding outwardly extending leads 65. The upper surface of the package includes an integrally molded opening 70 that directly permits contact to integrated circuit die 54.
In the embodiment shown in FIG. 7, the upper mold portion 61 includes a body 62 and a notch 63 extending downwardly from body 62. Upper mold portion 61 is brought into contact with lower mold portion 64 and clamped. Plastic encapsulating material is injected into cavity 68, forming the plastic package. One important drawback for this method is that integrated circuit die 54 may be crushed by the extremely high-pressure associated with such process, even if high-temperature silicon rubber, which is highly compressible, is used as the mold material. Another drawback of the above method is the extreme difficulty in aligning notch 63 to integrated circuit die 54.
A second embodiment of the '248 patent also employs injection molding to build an I.C. die package. As shown in FIG. 8, electrodes 85, a body 82 of dissolvable material, and an adhesive layer 83 on the underside of electrodes 85 are aligned over and positioned onto the integrated circuit die 84 that, in turn, has been secured and connected to lead frame 80. The above structure is positioned within a conventional integrated circuit package injection mold (not shown). After removal from the injection mold, the structure is positioned in a bath 86 containing a liquid solvent 87, so that body 82 of dissolvable material is dissolved away, leaving an opening to the underlying portion of the integrated circuit die 84. This method is undesirable because it requires additional processing steps and is thus more complicated and costly.
The molded plastic technology, in general, has several drawbacks. For example, the molded plastic technology incorporates various processes following the wire bonding procedure that may detrimentally affect the bonding integrity. These processes include sealing, which involves high-pressure injection-molding and cooling/heating steps, and the bending of the leads to achieve desired lead configurations, whereby bonding wire movement, breakage, and/or shorting can all result. Moreover, the encapsulation process is limited to the use of molding compounds with low thermal conductivity that can perform poorly.
Additionally, the use of lead frames during the manufacturing of semiconductor packages has many disadvantages. First, the dies from which conventional lead frames are stamped can be very expensive because of the number of intricate features of the circuit involved and the amount of material that must be handled. Moreover, the manufacturing tolerances required in stamping the larger sizes of necessary elements of the circuit cause the stamping of lead frames to be a low-yield process. Also, packages that incorporate lead frames are typically tested after die placement at a point so late in the manufacturing process that if the package turns out to be defective, any value that may have been added is rendered useless.
Additionally, lead frames typically limit the die placement process to procedures such as single-row peripheral pad bonding or tape automated bonding, thereby limiting die placement options and flexibility. Furthermore, once a conventional semiconductor package is completed, it is very difficult, if not impossible, to carry out repairs on one or more of the components of the package. In general, for conventional packaging technology, as the number of required leads increases, based on increases in the speed and functionality of the relevant die, so does the size of the lead frame, increasing its manufacturing and tooling costs and decreasing its efficiency due to the increased distances the signal must travel.